Federal appeals court judge Merrick Garland, receives applauds from President Barack Obama and Vice President Joe Biden as he is introduced as Obama's nominee for the Supreme Court during an announcement in the Rose Garden of the White House, in Washington, Wednesday. (AP/Pablo Martinez Monsivais)

By nominating an uncontroversial judge, President Barack Obama gives Republicans an unwelcome election-year proposition: Approve him or risk letting Hillary Clinton or Donald Trump pick a Supreme Court justice you might like even less.

Obama's selection of appellate judge Merrick Garland landed with a bang the morning after primaries in Florida, Ohio and other key states made clear that Clinton and Trump will be their parties' presidential candidates, barring extraordinary circumstances.

Republican leaders still insist that the next president should choose the replacement for the late Antonin Scalia, the influential conservative and high court's most provocative member. Senate Majority Leader Mitch McConnell called it "an issue where we can't agree."

But Obama's successor could be a Democrat.

Republicans recognize that if Clinton wins the presidency, she could nominate someone far more liberal than Garland, who is regarded as a centrist. At the same time, the Republican establishment is extremely wary of the unpredictable Trump and desperate for an alternative presidential candidate.

A Democratic victory at the presidential level could be accompanied by a return of the Senate to Democratic control, further complicating Republicans' ability to prevent Democrats from getting their way. Republicans are fighting their toughest Senate races this year in states like New Hampshire, Wisconsin and Illinois, where Democrats are hoping independent-minded voters will be turned off by the Republicans' hardline position.

Not only do many Republicans fear a Trump nomination would hurt them, many conservatives doubt Trump is really one of them. Trump's suggestion Wednesday that he is mainly taking policy advice from "myself" added uncertainty about what kind of justice he would pick.

Democrats are already pressuring Republicans to change their stance on filling the Supreme Court seat. Within minutes of Garland's nomination, Senate Democratic leader Harry Reid accused Republicans of "blindly taking their marching orders" from Trump.

Dan Pfeiffer, Obama's former senior adviser, said the combination of Garland's profile and a looming Trump nomination had increased pressure on Senate Republicans.

Clinton's string of recent primary wins added another element of intrigue to the Supreme Court fight. If she's elected and Democrats recapture the Senate, Republicans might choose to confirm Garland before Clinton takes office.

"I think that's self-evident," said Arizona Sen. Jeff Flake, one of the only Republicans agreeing to even meet with Obama's nominee. "Between him and somebody that a President Clinton might nominate, I think the choice is clear."

Clinton, in her response to Obama's nomination, praised Garland as a "brilliant legal mind" and urged senators to confirm him.

No Republican has yet broken with McConnell by calling for Garland to get a vote, though Sen. Susan Collins, a Maine moderate, called for a hearing.

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Associated Press writers Donna Cassata, Alan Fram and Kathleen Hennessey contributed to this report.